1. Field of the Invention
The present invention relates generally to a method of deagglomerating ceramic powder to have a uniform particle size; a deagglomeration mill used for the above method; and a method of preparing highly dispersed ceramic slurry using the deagglomerated ceramic powder.
More specifically, the present invention relates to a method of deagglomerating ceramic powder so as to inhibit agglomeration and achieve a uniform particle size; a deagglomeration mill for use in the above method; and a method of preparing highly dispersed ceramic slurry using the deagglomerated ceramic powder.
2. Description of the Related Art
Fabrication techniques of multilayer ceramic capacitors (hereinafter, abbreviated as ‘MLCC’) have recently trended toward miniaturization and ultrahigh capacitance, which can be realized by thinning an inner electrode and a dielectric layer and stacking more layers.
In particular, with the aim of manufacturing a multilayer structure having ultrahigh capacitance, a dielectric material, such as BaTiO3, MgO, MnO2, V2O5, Cr2O3, Y2O3, rare earth elements, and glass frit, constituting the dielectric layer, should be made fine. Also, slurry preparation to disperse fine particles is required to minimize the effects of high electric fields due to the formation of a thin dielectric layer of 3 μm or less so as to ensure electric reliability.
However, the formation of the fine particles causes high surface area, thus increasing the driving force of a sintering process. Eventually, rapid particle growth occurs.
In the fabrication of MLCCs having ultrahigh capacitance, BaTiO3, which comprises most of the starting materials, is used having the particle sizes of 0.2, 0.15 and 0.1 μm.
These particles agglomerate mostly during a particle synthesis process such as a hydrothermal process, an oxalate process, hydrolysis or solid-state synthesis, and during a heat treatment process to ensure consistent particle size and crystallinity and remove impurities.
Meanwhile, a chip is generally fabricated by mixing the BaTiO3 powder with a ceramic additive, an organic solvent, a plasticizer, a binder, and a dispersing agent to prepare a slurry using a basket mill, followed by forming, laminating and compressing the slurry.
A conventional slurry preparation process is illustrated in FIG. 1. That is, BaTiO3, the ceramic additive, the solvent and the dispersing agent are weighed and mixed, followed by primary milling for 12–24 hours, to obtain a pulverized product. Thereafter, the pulverized product is mixed with the binder, the plasticizer and the solvent, followed by secondary milling for about 12 hours and then filtering. At this time, however, reagglomeration and viscosity become high, and thus, it is difficult to perform the filtration process. After the filtration process, the mixed state of the slurry is analyzed using PSA (Particle Size Analysis), SEM (Scanning Electronic Microscopy) and EPMA (Electron Probe Micro Analysis).
As for conventional slurry dispersion, in cases where fine powder is dispersed using a basket mill or bead mill, soft and hard agglomerations of BaTiO3 are difficult to remove. The two mill media are zirconia and yittria zirconia, with a size of 0.6–1 mm.
The fine ceramic powder should be deagglomerated for high dispersion and grain growth control, before being used for the preparation of ceramic slurry. This is because an additive is not uniformly dispersed into neck-formed powders, and also, particles agglomerated during the heat treatment function as one crystal particle, and thus, it is impossible to form a dielectric thin film layer. As well, the reliability decreases. Therefore, the ceramic powder, for example, BaTiO3 powder, should be deagglomerated.
Specifically, in cases where BaTiO3 is finely particulate, agglomeration of the particles results in a wide particle size distribution. For example, 0.2 μm BaTiO3 has a particle size distribution of a D50 of 0.48 μm and a D90 of 1.0 μm. However, after such particles are subjected to deagglomeration treatment, the particle size thereof can be lowered to a D50 of 0.25 μm and a D90 of 0.45 μm, with narrow dispersion. That is, a uniform particle size can be ensured. By such a deagglomeration process, deagglomeration of the ceramic powder and attainment of a uniform particle size of the powder are realized.
Turning now to FIG. 2, a conventional bead mill 100 used for deagglomeration of ceramic powder is illustrated. The conventional bead mill 100 includes a hollow cylindrical mill cover 101 having an inlet 102 to load a mixture of ceramic powder and solvent into the mill cover 101, an outlet 104 to discharge the deagglomerated mixture therefrom, and a plurality of beads 114 therein. Further, a main shaft 110 is longitudinally disposed in the mill cover 101 to act as a central rotation shaft. Also, a driving means 120 for rotating the main shaft 110 is operatably connected thereto.
The driving means 120 is commonly exemplified by an electric motor, and is connected to the main shaft 110 through a power transfer belt 122 and pulleys 124a and 124b. Moreover, a plurality of circular discs 126 is mounted on the main shaft 110 to be rotated therewith. Each disc 126 has a plurality of through holes 126a so that the beads 114 can act to apply an action force to the ceramic powder upon rotation of the disc 126.
In the conventional bead mill 100 or basket mill as shown in FIG. 2, the mill cover 101 has a diameter (H) of 185 mm and a length (L) of 463 mm, and thus, a ratio of diameter to length of the hollow cylindrical mill cover 101 is 0.5 or less. That is, the bead mill 100 is relatively long relative to its diameter. In addition, the beads 114 have diameters of 0.65 mm or more. In the cases of using the conventional bead mill 100, the BaTiO3 powder is deagglomerated while having a long retention time in the bead mill 100. Hence, a high impact force and a high shear force are applied to the ceramic powder, and the ceramic powder particles are primarily pulverized, resulting in the generation of larger amounts of fine particles having sizes of 0.001–0.01 μm.
Since the fine particles have a particle size distribution with wide dispersion, the ceramic slurry having the above particles causes particle growth in the dielectric material after sintering, and thus, abnormal particle growth occurs.
Upon using the ceramic powder obtained by the conventional bead mill 100, it is difficult to separate the agglomerated particles from each other, and as well, the powder has a wide particle size distribution due to the pulverized primary particles. In particular, the pulverized fine powder having a wide size distribution acts as a driving force for abnormal particle growth during the heat treatment, and hence, negatively affects the electric reliability and dielectric properties.
In the MLCC having ultrahigh capacitance, the slurry is prepared by increasing the amount of an organic solvent, such as toluene, ethanol or butanol, to reduce the viscosity of the slurry, and is then filtered using a filter having a size of 1–2 μm to obtain a final slurry.
However, the slurry preparation method as mentioned above is disadvantageous because the solid content of the powder decreases while the content of the polymer increases, thereby generating larger amounts of residue and causing high constriction when burning out the binder. Also, the size of the filter is limited by the unmelted binder, and the lifetime of the filter is shortened.